Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into useful hydrocarbons.
This second step, the preparation of hydrocarbons from synthesis gas, is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). Fischer-Tropsch synthesis generally entails contacting a stream of synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons.
More specifically, the Fischer-Tropsch reaction is the catalytic hydrogenation of carbon monoxide to produce any of a variety of products ranging from methane to higher alkanes and aliphatic alcohols. Research continues on the development of more efficient Fischer-Tropsch catalyst systems and reaction systems that increase the selectivity for high-value hydrocarbons in the Fischer-Tropsch product stream.
There are continuing efforts to design reactors that are more effective at producing these desired products. Product distribution, product selectivity, and reactor productivity depend heavily on the type and structure of the catalyst and on the reactor type and operating conditions. Catalysts for use in such synthesis usually contain a catalytically active metal of Groups 8, 9, or 10 (in the New notation of the periodic table of the elements, which is followed throughout). In particular, iron, cobalt, nickel, and ruthenium have been abundantly used as the catalytically active metals. Cobalt and ruthenium have been found to be most suitable for catalyzing a process in which synthesis gas is converted primarily to hydrocarbons having five or more carbon atoms (i.e., where the C5+ selectivity of the catalyst is high).
Originally, the Fischer-Tropsch synthesis was operated in packed bed reactors. These reactors have several drawbacks, such as difficulty of temperature control, which can be overcome by using gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated reactors, sometimes called “slurry reactors” or “slurry bubble columns,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are converted to gaseous and/or liquid products. If gaseous products are formed, they enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid by passing the slurry through a filter that separates the liquid from the catalyst solids, and then separating the liquids.
A known problem in slurry reactors, however, is that water is continuously formed during Fisher-Tropsch synthesis in the reactors. This is known to limit conversion and prematurely deactivate catalyst systems such as iron and cobalt-based Fisher-Tropsch catalysts through an oxidation mechanism. As is well known in the art, a high water partial pressure correlates to a high deactivation rate. This is detrimental to the overall system performance, since two requirements for a successful commercial application of cobalt-based Fischer-Tropsch catalysts are a high conversion and, for middle distillates production, a high wax selectivity (or a high alpha value).
For any given cobalt-based catalyst, along with the H2/CO ratio and the reaction temperature, the total pressure is a parameter that has a direct influence on the wax selectivity, in that a higher pressure will result in a higher wax selectivity. However, a higher total pressure (at any given degree of conversion) also correlates to a higher water partial pressure and therefore a higher deactivation rate. Therefore, if reactors are operated at conditions conducive to higher alpha values, conversion will necessarily have to be low to avoid premature catalyst deactivation due to water. A low conversion is undesirable, however, because it results in higher capital investment and operating costs. Additionally, for iron-based catalysts, the water not only has a negative effect on the catalyst deactivation rate, but it also inhibits the rate of reaction.
The water partial pressure is therefore a constraint that will not allow the realization of the kinetic and/or wax selectivity potential of iron and cobalt-based Fisher-Tropsch catalysts. Therefore, in order to improve the efficiency of Fischer-Tropsch reactors using iron and cobalt-based catalyst systems, there exists a need for a method to remove water formed during Fisher-Tropsch synthesis.